1. Field of the Invention
The present invention relates to an idle speed control device for an engine which is equipped with an idle speed control valve for controlling the engine speed during an idle operation. More specifically, the present invention relates to an idle speed control device utilizing a two-solenoid rotary type idle speed control valve and is capable of maintaining the engine idle speed within an appropriate range even when one of the solenoids fails.
2. Description of the Related Art
An idle speed control device is used for maintaining the engine speed at a predetermined target value during the idle operation regardless of changes in engine temperature and engine load. The idle speed control device is usually equipped with an inlet air bypass passage connecting the portions of the inlet air passage upstream and downstream of the throttle valve, and an idle speed control valve for controlling the airflow passing through the inlet air bypass passage. The idle speed control device adjusts the engine speed by controlling the amount of the inlet air supplied to the engine using the idle speed control valve regardless of the degree of opening of the throttle valve during the engine idle operation.
Usually, a stepper motor is used for the actuator of the idle speed control valve and the degree of opening of the idle speed control valve is controlled by adjusting the driving pulse signal supplied to the stepper motor. Therefore, when a failure of the field coil in any phase of the motor occurs, such as a disconnection or ground of the coil, the engine idle speed cannot be controlled precisely.
Japanese Unexamined Patent Publication (Kokai) No. 3-57857 discloses a control device for a stepper motor which can control the motor even when the winding of one of the phases of the motor is failed. The device in JPP '857 detects the failure of the windings of the motor from the control signal of the drive transistors connected to the windings of the respective phases. When a failure of the winding of one of the phases occurs, the device cuts off the supply of the drive pulse to the failed winding and controls the motor using the remaining windings. The device in JPP '857 maintains the operation of the stepper motor at a nearly normal level when one of the windings fails by supplying the drive pulse to only the remaining windings.
An idle speed control valve having an actuator other than a stepper motor, such as a two-solenoid rotary type idle speed control valve, is also used for an idle speed control device. The two-solenoid rotary type idle speed control valve has two solenoids for controlling the degree of opening of the valve. In the two-solenoid rotary type idle speed control valve, when electric current is supplied to the solenoids, one of the solenoids urges the idle speed control valve to open, and the other solenoid urges the idle speed control valve to close. The degree of opening of the idle speed control valve is controlled by adjusting electric current supplied to the two solenoids in such a manner that the force urging the valve to open and the force urging the valve to close are balanced at a desired valve position. The two-solenoid rotary type idle speed control valve has advantages compared with the stepper motor type idle speed control valve in that it has simpler construction and quicker response.
However, the two-solenoid rotary type idle speed control valve has also the disadvantage that the valve may be maintained at a fully opened position or fully closed position when one of the solenoids fails. For example, when the closing solenoid is disconnected, the valve is maintained at the fully opened position when the opening solenoid is activated. On the other hand, when the opening solenoid is disconnected, the valve is maintained at the fully closed position when the closing solenoid is activated. Therefore, if one of the solenoids fails, the idle speed of the engine may become excessively high (when the valve is maintained at the fully opened position), or excessively low (when the valve is maintained at the fully closed position), and the latter may cause a stall of the engine.
Further, the stepper motor can be operated in a nearly normal manner without changing its control method according to the type of the failure of solenoid. As stated in JPP '857, the stepper motor can be controlled substantially normally even when one of the windings fails, by activating the remaining windings in a manner similar to their normal operation regardless of the type of the failure of the winding, i.e., regardless of whether the winding has been disconnected or short-circuited.
In the two-solenoid rotary type idle speed control valve, the operation of the valve is completely different depending on the type of failure of the solenoid as explained later in detail. Therefore, in order to prevent excessively high idle speed or an engine stall caused by excessively low idle speed, the control mode of the remaining solenoid must be changed according to the type of the failure of the other solenoid. However, since it is difficult to exactly determine the type of failures of the solenoid in some cases, it is difficult to control the two-solenoid rotary type idle speed control valve properly when one of the solenoids fails.